1. Field of the Invention
The invention relates to pressure measurement apparatuses and pressure and/or force measurement devices, in particular for high pressures or forces, whose housing contains among other things, a pressure or force measuring cell, e.g. a piezoresistive or capacitive measuring cell. A capacitive pressure measuring cell is essentially comprised of a base body (e.g. ceramic) and a membrane (e.g. ceramic) connected to it and electrodes, whose spacing change due to membrane deflection achieves a capacitance change. A seal between the measurement device housing and the pressure measuring cell prevents the penetration of pressure medium into the inner chamber of the housing.
2. Discussion of Background Information
Rubber elastic O-rings made of elastomers are used for seals of this kind, e.g. if the pressure measuring cell is comprised of a non-metallic material (e.g. ceramic) and the housing is comprised of metallic material (e.g. V4A).
An example of this is capacitive pressure measurement devices of the Cerabar Series of the PMC731-R12S1E19Y4 type from the Company Endress & Hauser GmbH (pressure measurement range 0-40 bar, cylindrical ceramic pressure measuring cell). Like all other known pressure measurement devices with non-metallic membrane or pressure measuring cell, they have a high housing stability in the region of the pressure measuring cell and the elastomer seal, because of the housing thickness of several millimeters. Because of the positive corrosion resistance of the ceramic membrane, separating membranes and pressure averagers are actually not necessary. The rubber elastic seals, however, are susceptible to wear. As a result, therefore, waved stainless steel membranes are frequently used.
The above mentioned Endress & Hauser pressure measurement device is embodied as flush in front, i.e. the membrane surface toward the medium is disposed very near to the housing end, while in pressure measurement devices which are not flush in front, the pressure medium travels via a more or less long conduit of the solid housing, reaching the pressure measuring cell (e.g. see German Patent Application P 44 16 978.7, FIG. 1, and the reference "Kapazitiver keramischer Drucksensor hoher Stabilitat fur die Prozessmesstechnik" High Stability Capacitive Ceramic Pressure Sensors for Process Measuring Technology!, by Drewes, Friedrich, Hegner, Klahn and Schmidt).
In flush front pressure measurement devices, a smaller dead space can be achieved, which is important for use of pressure measurement devices in food, chemical, pharmaceutical, and paper manufacturing. A small dead space means few gaps and hollow spaces, which in the event of a medium change, could lead to difficulties in the removal of the old medium (e.g. see the pamphlet "Totraumfreie Instrumentierung--Inline-Kontroll- und Messtechnik" Dead Space-free Instrumentation--In-line Control and Measurement Technology! from the Tuchenhagen company).
These kinds of apparatuses made up of pressure measuring cell, housing, and O-ring are not without problems as regards tightness or service life of the O-ring, even at average pressures (20-40 bar). This has been disclosed, for example, in the reference "Ubersicht uber verschiedene Aufnehmerprinzipien fur die elektrische Druckmessung" Overview of Various Sensor Principles for Electrical Pressure Measurement! by R. Hellwig, p. 2.5, section 2.3.2. Since as a rule, no static continuous loading and continuous deforming occurs, but a dynamic loading, which, due to the movement and deforming of the O-ring, can lead to frequent exceeding of strength and stretchability limits and consequently to the destruction of the O-ring. When there is constantly repeating deforming, the material is damaged as a result of internal friction, by means of which first small tears occur, which can grow and eventually lead to breakage.
O-rings for sealing purposes are generally made of widely varying materials. Primarily, elastomers are used (e.g. acrylonitrile-butadiene rubber and acrylate rubber). Sealing elements of thermoplastic materials differ according to the base materials used. In many instances, they can be varied by introducing particular additives and can thus be purposefully matched to employment purpose of the part to be manufactured.
Elastomers are wide-meshed, temperature stable, cross-linked polymers which do not become liquid from a low 20.degree. C. up to the temperature region of chemical decomposition, but can be reversibly deformed in a rubber elastic manner, in a manner that is to a large extent independent of temperature.
A body is elastic if, after a forced deforming, it re-assumes its original shape relatively rapidly (e.g. a metal spring). A body which maintains its deformation is plastic or viscous (e.g. kneaded rubber). A viscoelastic body is both at the same time. It is characteristic of viscoelastic behavior that when springing back, the original state is reached not immediately after release, but only gradually, depending upon the conditions.
This property is characteristic of rubber elastic O-ring seals. Just like deformability, elasticity is dependent upon temperature and primarily upon the chronological course of the deformation process. Temperature load, temperature change load, pressure load, and pressure change load decisively determine the service life of rubber elastic O-rings.
As described above, their use for sealing pressure measuring cells and housings is already problematic at average pressures if relative movements of the parts to be sealed occur (membrane/pressure measuring cell and housing). This kind of relative movements can be caused by temperature change in combination with differing thermal expansion coefficients of the parts to be sealed and by pressure changes which lead to undesirable movements (axial and radial) of the membrane surface or pressure measuring cell surface in its edge region, (e.g. in the region of the seal, see German Patent Application P 44 16 978.7). This movement is the greatest at the membrane edge and is particularly critical in the case of capacitive pressure measurement devices without pressure averagers, which devices have a relatively thick ceramic membrane. The magnitude of this movement is a function of the pressure, the membrane diameter, the inner diameter of the spacer, the thickness of the pressure measuring cell (even the base body, which is quite stable, has a deflection), as well as the mechanical properties of the membrane, spacer, and base body. Especially when employed in the food industry, the use of relatively soft rubber elastic O-ring seals is problematic due to the high temperature shock load. In principle, harder seals can in fact also be used, which as a rule also have a better chemical resistance, but then, a stronger O-ring must be used, which impairs frontal flushness (larger dead space) and constitutes a larger abrasion surface for abrasive mediums.
The durable tightness is important not only between the pressure measuring cell and the housing of the pressure measurement device, but also between the flange and the pressure measurement device. A flange from the WIKA company is known for the flush front screw connection of a low pressure measurement device. The flange and pressure measurement device have a metallic sealing cone. The thickness of the conical sealing part is 5 mm. This embodiment is disadvantageous in that it can be problematic with regard to tightness at higher pressures. It is also disadvantageous that the force with which the pressure measurement device presses against the conical part of the flange when screwed in can vary widely. If the force is too great, this can negatively influence the characteristic curve or the operability of the pressure measurement device.